我有 8Gb RAM，但系统报告 6.8 Gb，而在 BIOS 中，它显示 8Gb。我没有任何交换。我在 Lubuntu 18.04 64 位上。
sudo lshw -C memory
*-firmware
description: BIOS
vendor: AMI
physical id: 0
version: F.10
date: 05/17/2018
size: 64KiB
capacity: 15MiB
capabilities: pci upgrade shadowing cdboot bootselect edd int13floppy1200 int13floppy720 int13floppy2880 int5printscreen int9keyboard int14serial int17printer acpi usb smartbattery biosbootspecification netboot uefi
*-memory
description: System Memory
physical id: 9
slot: System board or motherboard
size: 8GiB
*-bank:0
description: SODIMM DDR4 Synchronous Unbuffered (Unregistered) 2400 MHz (0,4 ns)
product: HMA81GS6AFR8N-UH
vendor: Hynix
physical id: 0
serial: 825AC392
slot: Bottom - Slot 1 (left)
size: 8GiB
width: 64 bits
clock: 2400MHz (0.4ns)
*-bank:1
description: SODIMM [empty]
product: Unknown
vendor: Unknown
physical id: 1
serial: Unknown
slot: Bottom - Slot 2 (right)
*-cache:0
description: L1 cache
physical id: b
slot: L1 - Cache
size: 192KiB
capacity: 192KiB
clock: 1GHz (1.0ns)
capabilities: pipeline-burst internal write-back unified
configuration: level=1
*-cache:1
description: L2 cache
physical id: c
slot: L2 - Cache
size: 1MiB
capacity: 1MiB
clock: 1GHz (1.0ns)
capabilities: pipeline-burst internal write-back unified
configuration: level=2
*-cache:2
description: L3 cache
physical id: d
slot: L3 - Cache
size: 4MiB
capacity: 4MiB
clock: 1GHz (1.0ns)
capabilities: pipeline-burst internal write-back unified
configuration: level=3
free -h
total used free shared buff/cache available
Mem: 6,8G 1,1G 4,4G 63M 1,3G 5,1G
Swap: 0B 0B 0B